1. Field of the Invention
The present invention relates to an apparatus for aligning two objects with each other and particularly, an apparatus for aligning a mask and wafer with each other prior to the exposure of the wafer to the pattern of semiconductor integrated circuit on a photo-mask or reticle, which will be referred to simply by the term "mask" hereinafter.
2. Description of the Prior Art
Semiconductor producing process includes sequential steps for projecting plural patterns to a wafer to form a semiconductor integrated circuit. In order to obtain the accurate positioning of a pattern on the wafer which has been exposed to a pattern or patterns in the previous step(s), a mask having that pattern thereon must be aligned with the wafer with high accuracy. Typically, such an alignment is automatically accomplished by photoelectrically detecting alignment marks on the mask and wafer with the resulting signals being used for the automatic alignment.
There is known a mask change mechanism comprising a mask carrier containing a mask therein and means for automatically setting the mask in the mask carrier precisely at a predetermined position. Such a mechanism must have a function that the mask is preliminarily aligned with an exposure stage just at a predetermined position for setting. This is called "mask alignment".
U.S. Pat. No. 4,052,603 states that the precise setting of a mask is particularly important for an aligner of such a type that it is operated in step-and-repeat motion. This patent provides an extremely cumbersome alignment method.
One of the prior art alignment systems provides a mask having an alignment mark for realizing the mask alignment and another alignment mark for detecting an error in relative position between the mask and a wafer to align them with each other. These alignment marks are different from each other in shape. It is believed that this is because the respective alignments should be made by different optical arrangements. This prior art alignment system further includes different sensors for detecting the respective alignment marks.
The prior art alignment system has various disadvantages. The different shapes in the alignment marks require a corresponding number of operational programs for detected signals. Different programs also are required to drive the different sensors. Additional sensors including the respective precise optical systems will make the aligner more expensive.
If any alignment mark is not in the field of view of the corresponding sensor upon alignment, a stage holding a wafer or mask or a detection optical system is moved to search that alignment mark. In the prior art, there is taken a searching method in which the stage or detection optical system are first moved to the margin of the searching range and then driven inwardly to search the alignment mark in the searching range.
In such a searching method, however, time is required to move the stage or detection optical system to the margin of the searching range. The stage is suitable for fine motion in its precisely positioning operation but not for rapid motion. This is further required because the search is initiated from the margin of the searching range in which the presence of the alignment mark is less possible. It is believed that the presence of the alignment mark is more convenient in the central setting position even if any mechanical error exists in setting.